Small scale, energy-efficient desalination demoed

Combining a voltage difference with a selectively permeable membrane can …

Right now, estimates are that a few hundred million people suffer from limited access to fresh water, and population growth and climate change are expected to rapidly exacerbate that problem. Many of these people have access to brackish or salty water, but desalinization plants only work efficiently on the large scale, with an attendant infrastructure. A paper released by Nature Nanotechnology on Sunday describes a microfluidic approach to desalinization that is roughly as energy-efficient as a large-scale plant, but compact enough that it could operate as a small, battery-powered device.

The basic idea behind the device is extremely simple: send a flow of salty water down a channel with a Y-shaped junction, and convince all the charged items, whether they're salt ions or cells, to take the left turn. That leaves a flow of relatively pure water running down the right fork.

Convincing everything to make a left turn requires a technique termed ion concentration polarization. The right-hand fork of the device has a small channel covered with a membrane called Nafion that only admits small, positively charged ions (called cations). There's also a voltage difference across that channel that repels the cations. Instead of backing everything up right at the Nafion membrane, this setup actively repels all charged particles. Placing it at the mouth of the right-hand channel neatly forces all the charged materials to the far-left-hand wall of the device, where it will be carried down the left turn of the device. The water that flows down the right-hand side will be free of charged materials—meaning salt-free.

Membranes are currently used in desalinization processes, such as reverse osmosis, but those techniques require a great deal of care to be taken in order to avoid clogging the pores of the membranes. In the technique outlined here, most of the material is actively repelled before it gets anywhere near the membrane, which the authors suggest should greatly expand its usable life span.

The authors tested a small version of the device using sea water obtained from a beach in Ipswich, Massachusetts (they were based at MIT), which was passed through a filter to get rid of particulates. To monitor the flow and confirm that biological materials would go along for the ride (most biological materials carry a charge under voltage), they spiked the sea water with red blood cells and a fluorescent molecule.

Under a bit of pressure, the material would flow evenly down both the left and right channels. But, almost as soon as the voltage is applied, the active repulsion kicked in, and an ion depletion boundary formed just upstream of the right-hand channel. As a result, the fluid that flowed down the right-hand passage was clear (the process was so efficient that about half the material flowing down the left-hand passage was also clear.

The material collected from the desalinized channel had about 180mg/l of ionic material in it; that's a significant improvement from the 30,000mg/l in the source material, and it is less than a third of the upper limit for potable water. The power requirements are estimated at between four and five Watt-hours for each liter of water, which is in line with large commercial desalinization techniques. The water requires so little force to move through the channels in this new system that the hardware could simply be gravity fed.

Leaving the realm of implementation, the authors consider how to scale the device up by expanding the diameter of the channels and adding multiple channels to a single device. They come up with a six-to-eight inch chip (<20cm) that would be capable of producing over 200ml of fresh water every minute. By combining the Nafion membrane and one of the electrodes, it's also possible to cut down on the energy required, but in any case, it should require far less power than current commercial devices, which aren't nearly as portable and require in the neighborhood of 100W-hr/liter. (For context, it's possible to pick up 100W solar panels on Amazon for roughly $300; they cover roughly a square meter.)

There are a couple of major caveats to this work. For starters, the filtering technique doesn't get rid of an neutral organic materials, which means that, although the water that comes out is relatively salt-free, it may not be safe to drink until it's also passed through a material like activated carbon. The other issue is that concentrating calcium ions tends to cause them to precipitate, so the authors added sodium hydroxide to the sea water first, which precipitated out the calcium before it hit the filter. Neither of these steps are show-stoppers, but they would add to the complexity of the device.

In any case, the authors don't think that the hardware's ready to replace large-scale facilities. Instead, they suggest that it might make sense in "disaster- and poverty-stricken areas," where there's either a need for temporary desalinization, or access to the infrastructure needed to support industrial desalinization is unlikely to arrive any time soon.

[q]the process was so efficient that about half the material flowing down the left-hand passage was also clear[q]

Uh, wait. So what HAPPENED to the bad stuff? Does it just build up in the filter? I would think you'd want the left-hand channel to be less clear than the incoming material (ie, higher concentration of bad stuff), as it was removed from water flowing through the right-hand channel?

By combining the Nafion membrane and one of the electrodes, it's also possible to cut down on the energy required, so the new device should theoretically require less than 100W-hr/liter. Currently, it's possible to pick up 100W solar panels on Amazon for roughly $300; they cover roughly a square meter.

I don't know if the paper or author made that jump, but 100W solar panels extremely rarely put out 100W (all but perfect conditions - 25C, 1000W/sq meter solar irradiation, etc). A 125W panel would be suitable to put out 100W in sunny conditions with less irradiation and higher temps, and costs $500 (buying in bulk would be a significant discount - closer to $3/W).

Uh, wait. So what HAPPENED to the bad stuff? Does it just build up in the filter? I would think you'd want the left-hand channel to be less clear than the incoming material (ie, higher concentration of bad stuff), as it was removed from water flowing through the right-hand channel?

The "bad stuff" is just concentrated salt. It's in a separate channel that you can just have flow back into wherever the water came from in the first place.

doormat wrote:

Quote:

By combining the Nafion membrane and one of the electrodes, it's also possible to cut down on the energy required, so the new device should theoretically require less than 100W-hr/liter. Currently, it's possible to pick up 100W solar panels on Amazon for roughly $300; they cover roughly a square meter.

I don't know if the paper or author made that jump, but 100W solar panels extremely rarely put out 100W (all but perfect conditions - 25C, 1000W/sq meter solar irradiation, etc). A 125W panel would be suitable to put out 100W in sunny conditions with less irradiation and higher temps, and costs $500 (buying in bulk would be a significant discount - closer to $3/W).

Yeah, I realized that, but figured I'd provide a rough ballpark. In any case, the 100W for sale was actually two 50W, so you could just add whatever you need to that pretty easily.

The "bad stuff" is just concentrated salt. It's in a separate channel that you can just have flow back into wherever the water came from in the first place.

So there's a fourth output on the Y junction?

As I read it, it's salty water into the bottom of the Y, then the Nafion with the charge on the right hand path pushes salt away from it, and clear water flows through it, and the rest would therefore take the left hand path. But if the left hand path is ALSO half clear, there's either extra water coming from somewhere, or salt disappearing? Or I'm just really confused.

As I read it, it's salty water into the bottom of the Y, then the Nafion with the charge on the right hand path pushes salt away from it, and clear water flows through it, and the rest would therefore take the left hand path. But if the left hand path is ALSO half clear, there's either extra water coming from somewhere, or salt disappearing? Or I'm just really confused.

The left hand path is half clear just because the salty half of it doesn't remix with the desalinized portion immediately. Given time, they would, but the flow through the device isn't turbulent, so not a lot of mixing happens.

Sorry for the confusion - that section was just meant to suggest that the system was efficient enough that there's a significant margin of error.

This would be awesome for relief efforts. But I also wonder how it would handle industrial pollutants. EG: after Katrina, New Orlean's flood waters were mixed with a lot of fuels and other junk. I'm assuming this wouldn't help filter that out ... only things that would be considered a "salt" that formed some kind of ionization in the water, correct?

The power requirements are estimated at between four and five Watt-hours for each liter of water, which is in line with large commercial desalinization techniques ... it should require far less power than current commercial devices, which aren't nearly as portable and require in the neighborhood of 100W-hr/liter.

Can someone explain this to me? It seems contradictory. If it's part of the "fixing it now" but is not yet fixed, my apologies.

OK, so let's see. 200mL/min, and about 5Wh/L, so its power requirements are about 1Wh/min, or 60W. So a 100W solar panel, even without peak efficiency, could keep up with it without needing a battery. Nice.

This would be awesome for relief efforts. But I also wonder how it would handle industrial pollutants. EG: after Katrina, New Orlean's flood waters were mixed with a lot of fuels and other junk. I'm assuming this wouldn't help filter that out ... only things that would be considered a "salt" that formed some kind of ionization in the water, correct?

Desalination is just but a part of the process of making potable water. The article mentions initial filtering for particulates and the use of activated carbon. The activated carbon would remove the organic pollutants. Heavy metals and arsenic, on the other hand...

@microbrew For some situations this could be useful. There are really cheap filtering or chloride based solutions which will take care of most bacteria/virus. Most situations do not have major issues with heavy metal or arsenic, and even if you have arsenic in the water it is generally only an issue if you drink it for years.

It strikes me that this might marry rather nicely with more conventional RO systems as an inexpensive method of pre-treating the water flow and extending the operational life of the conventional membranes.

Either way, fresh water is a big problem for the world, and this is interesting work!

When the Ca ions precipitate out, what form do they take? If they're Ca CO3 (calcium carbonate), then this technique has another potential application. See, the reason why CO2 is accumulating in the atmosphere is because the process in the geological carbon cycle that removes it is very slow. That process involves, essentially, locking up the CO2 into various carbonate rocks (ie limestone). If this technology can speed that up greatly then it has the potential to unlock real long term carbon sequestration. It wouldn't be a magic bullet, mind, but removal of CO2 from the atmosphere is really the best solution to a problem which originates from the addition of CO2 to the atmosphere.

We still want to transition away from fossil fuels, however, considering that peak oil appears to be around the corner, and peak coal is likely to occur within the lives of our children or grandchildren.

So, how does it compare to reverse osmosis? If it can be scaled up to be cheaper than reverse osmosis, I'd imagine semiconductor manufacturers would love to incorporate this into their deionized water plants and get rid of the reverse osmosis stuff, especially if it involves less care and maintenance.

Ok, checked the paper and it precipitates out as calcium hydroxide (Ca(OH)2). Any chemists in the audience know whether the addition of significant CO2 to make carbonic acid would cause the precipitate to be Ca(CO3)? Because even if this doesn't work at atmospheric concentrations of CO2, this still could have application in the CCS (carbon capture and sequestration) area if it can be hooked up to a concentrated source of CO2 to produce limestone.